In recent years, the noise from power transformers poses problems as environmental pollution. The noise of power transformers is mainly caused by magnetostriction of a grain oriented electrical steel sheet used as an iron core material of transformers. To reduce the noise of transformers, it is required to reduce the magnetostriction of the grain oriented electrical steel sheet. An industrially advantageous solution is to cover the grain oriented electrical steel sheet with an insulation coating.
As properties required for insulation coatings for grain oriented electrical steel sheets, tension induced by a coating, moisture-absorption resistance, rust resistance, and lamination factor are mentioned. Among the properties, securing the tension induced by a coating is important for the reduction in the magnetostriction. Here, the tension induced by a coating refers to tension given to grain oriented electrical steel sheets by the formation of insulation coatings.
The coatings of grain oriented electrical steel sheets generally contain a ceramic forsterite coating formed by secondary recrystallization annealing and a phosphate-based insulation coating provided thereon. As a method for forming the insulation coating, techniques disclosed in Japanese Unexamined Patent Application Publication Nos. 48-39338 and 50-79442 are known. In these techniques, a treatment solution for insulation coating containing colloidal silica, phosphates, and chromium compounds (e.g., one or two or more members selected from chromic anhydrides, chromates, and dichromates) is applied to a steel sheet, and then the steel sheet is baked.
The insulation coatings formed by these methods have effects of improving the magnetostriction properties by giving tensile stress to grain oriented electrical steel sheets. However, the treatment solutions for insulation coating contain chromium compounds, such as chromic anhydrides, chromates, or dichromates, as components for maintaining favorable moisture-absorption resistance of the insulation coating, resulting in the fact that the treatment solutions for insulation coating contain hexachromium derived from the chromium compounds. Japanese Unexamined Patent Application Publication No. 50-79442 also discloses a technique of adding no chromium compounds. However, the technique is extremely disadvantageous from the viewpoint of moisture-absorption resistance. The hexachromium contained in the treatment solution for insulation coating is reduced into trivalent chromium by baking to be detoxicated. However, there arise problems in that various difficulties occur in handling in waste liquid treatment of the treatment solution.
In contrast, as a so-called chromium-free treatment solution for insulation coating for grain oriented electrical steel sheet not substantially containing chromium, Japanese Examined Patent Application Publication No. 57-9631 discloses a treatment solution for insulation coating containing colloidal silica, aluminum phosphate, and boric acid, and further containing one or two or more members selected from sulfates of Mg, Al, Fe, Co, Ni, and Zn. Moreover, Japanese Examined Patent Application Publication No. 58-44744 also discloses a treatment solution for insulation coating containing colloidal silica and magnesium phosphate and further containing one or two or more members selected from sulfates of Mg, Al, Mn, and Zn. However, the use of the treatment solutions for insulation coating of Japanese Examined Patent Application Publication Nos. 57-9631 and 58-44744 has caused problems in terms of tension induced by a coating and moisture-absorption resistance in a request to coating properties in recent years.
As methods for solving the problems of lack of tension induced by a coating, moisture-absorption resistance lack, and the like occurring when the treatment solution for insulation coating is rendered chromium-free, Japanese Unexamined Patent Application Publication No. 2007-23329 discloses a chromium-free treatment solution for insulation coating containing a dispersion liquid of a colloidal compound containing (I) colloidal silica, (II) phosphate, and (III) one or two or more metal elements selected from Fe, Al, Ga, Ti, and Zr.
However, according to our study, when the treatment solution for insulation coating described in Japanese Unexamined Patent Application Publication No. 2007-23329 is used, there are problems in that a surface free from stickiness is obtained immediately after baking, but stickiness arises during prolonged storage, such as one month or two months, and the moisture-absorption resistance is still insufficient.
Our steels and methods have been developed in view of the above-described present circumstances, and aim to achieve each of the following items:                preventing the reduction in tension induced by a coating and moisture-absorption resistance which poses a problem when a treatment solution for insulation coating is rendered chromium-free,        providing a chromium-free treatment solution for insulation coating for grain oriented electrical steel sheet capable of providing a grain oriented electrical steel sheet having excellent insulation coating properties, i.e., excellent tension induced by a coating, moisture-absorption resistance, rust resistance, and lamination factor, and        providing a method for producing a grain oriented electrical steel sheet having an insulation coating using the chromium-free treatment solution for insulation coating for grain oriented electrical steel sheet described above.        